The solitary tract nucleus (nTS), located in the dorsomedial medulla, is the principal sensory nucleus of the vagus and glossopharyngeal nerves. This cell cluster is known to be important in mediating autonomic reflexes that involve the cardiovascular system. A wealth of anatomic data indicates that a sizeable afferent pathway to the nTS arises from the medullary raphe nuclei (pallidus, obscuras and magnus). Both stimulation of these nuclei and microinjection into the nTS of the primary neurotransmitter of raphe neurons, serotonin, has been found to elicit decreases of blood pressure and heart rate and to induce high voltage, it appears likely that the pathway to the nTS mediates some of the effects of the medullary raphe on autonomic and electrocortical activity, such as may occur during the onset of slow-wave (synchronous) sleep. The present project was designed to investigate the interactions between the raphe and the nTS in vivo in the anesthetized rat by assessing the influences of the raphe on neuronal activity and the transmission of visceral sensory information within the nTS, as well as by defining the importance of the nTS in the elicitation of changes of hemodynamic and electrocortical activity brought about by stimulation of the raphe. This will be accomplished with the use of extracellular single unit recording techniques, hemodynamic and electroencephalographic monitoring and administration of drugs by microiontophoresis, micropneumophoresis and microinjection, as well as selective lesioning and hyperpolarization blockade within and outside brainstem areas of interest. This project will also involve extracellular studies in vivo in the anesthetized rat and both extracellular and intracellular studies in vitro in the superfused rat brain slice to provide physiological and pharmacological information regarding the effects of serotonin on neuronal activity and transmission visceral information within the nTS and on peripheral hemodynamics and electrocortical activity. Information will also be derived regarding the possible modulation of these effects by substance P or enkephalins, neuropeptides that have found within the nTS either in isolation or co- localized in synaptic terminals arise from raphe neurons. Finally, histological techniques will be used to reveal the distribution and morphology of nTS neurons that respond to afferent raphe input or application of the various identified raphe neurotransmitters.